1. Field of the Invention
The present invention relates to an ink-jet printing apparatus, and particularly relates to a nozzle head of an ink-jet printing apparatus of the type in which the volume in the pump chamber is suddenly changed responsive to electric signals such that ink particles are generated from the nozzle at a period corresponding to the electric signals.
2. Prior Art
A variety types of non-impact printing devices have already been proposed to make records such as characters on a recording medium such as recording paper by injecting small ink particles from a nozzle head.
According to the earlier ink-jet printers used for the apparatus of this type, the ink compressed by a compressor pump is injected from nozzles while being imparted with ultrasonic vibrations thereby to generate a beam (or a stream) of ink particles.
The ink particles continuously injected from the nozzles are electrically charged responsive to record-information signals, allowed to pass through an electrostatic field, deflected depending upon the amounts of electric charge possessed by the ink particles, and are permitted to reach predetermined positions on a recording paper.
On the other hand, the ink particles which are not used for effecting the recording are not electrically charged responsive to the recording signals. Hence, such ink particles are not deflected but are allowed to travel straight even after having passed through the electrostatic field, and are recovered by means of a gutter.
According to the abovementioned device, among the ink particles injected from the nozzles, the ink particles which do not participate in the recording amounts to as great as 3 to 10 times the amount of the ink adhered on the recording paper to effect the recording. Therefore, the ink particles which does not participate in the recording are recovered by means of the gutter as mentioned above, and are reused.
When the ink particles are recovered and reused, however, the quality of ink particles is often changed while they fly through the air, or dust and dirt are mixed into the ink, causing the nozzles to be clogged and making it difficult to maintain the reliability of the apparatus, unless any suitable devices are provided to prevent such defects.
In recent years, however, attention has been given to an inj-jet printer of the type of on-demand which produces ink particles from the nozzles only when the ink particles are needed.
For instance, U.S. Pat. No. 3,946,398 discloses an ink-jet printer of the type in which the ink in an ink tank is supplied to a nozzles head through a pipe, a pump chamber in the nozzle head is excited by means of an electrostrictive element, and the ink particles are injected from an orifice of the nozzle responsive to electric signals applied to the electrostrictive element such that the ink is adhered onto a recording paper.
According to the nozzle head of the type mentioned above, therefore, the ink is injected only at the time of effecting the recording by controlling the pulse voltage applied to the electrostrictive element, making it possible to improve such problems as the degradation of ink or the recovery of the unused ink. Besides, the ink can be introduced into a pump chamber from an ink reservoir during the step in which the wall of the pump chamber deformed by the electrostrictive element restores its shape, enabling the device to be constructed in a small size.
Further, by suitably determining the shape of nozzle holes of the nozzle head, the ink particles can be caused to fly in a predetermined direction. Accordingly, by arraying a plurality of such nozzles, a desired recording can be effected without the need of deflecting the ink particles way of electrostatic field.
The nozzle head employed by the above ink-jet printer is usually composed of a base plate, a covering plate and an electrostrictive element mounted on a position opposed to the pump chamber on the covering plate. Grooves of predetermined shapes will have been formed in the base plate. By placing the covering plate on the base plate, the ink reservoir, the pump chamber and the nozzle holes are formed as a unitary structure.
To attain good recording using such an apparatus, however, the diameter of ink particles must be selected to be about 100 .mu.m. For this purpose, the nozzle holes must have a size as small as about 50 to 100 .mu.m requiring very high degree of dimensional precision.
With the earlier apparatus in which the base plate and covering plate are stuck by means of an organic adhesive agent or soldering, however, the adhesive agent often entered into the nozzle holes causing the cross-sectional areas of the nozzle holes to be varied or resulting in the clogging of nozzle holes. Furthermore, it was very difficult to from orifices of a plurality of nozzles maintaining uniform cross-sectional area.
Further, through the study by the inventors of the present invention, it was revealed that the shape of the nozzles must be finished maintaining very high precision because of the reasons mentioned below.
(1) In general, the compressed progressive wave in ink in a capillary tube is easily affected by the stickiness of the ink with respect to the tubular wall. Once eddy currents are created on the tubular wall, sticky current is peeled off, and the ink flows in a zig-zag manner in the tube. Therefore, the ink particles do not fly constantly. Therefore, the wall surface in the nozzle through which the ink flows must be very smooth.
(2) The viscous flow of ink flowing through the tubular wall becomes unstable under a particular condition determined by a relation between the progressive speed of the compressed progressive wave and the cross-sectional area of the flow path of the nozzle. Particularly, when the cross-sectional area of the flow path is not constant in a direction in which the ink flows, a flow tends to develop along a portion of the inner wall of the nozzle making it difficult to fly the ink in a predetermined direction. Therefore, the flow path of the nozzle must have a constant cross-sectional area and must be straight.
(3) The ink particles are injected from the tip of the nozzle overcoming the surface tension of ink at the tip of the nozzle. Therefore, the size of the ink particles are greatly varied depending upon the cross-sectional area at the tip of the nozzle. Hence, the deviation of cross-sectional areas at the tips of each of the nozzles must be reduced as small as possible so that the area of each dot recorded by the ink particles is confined within a predetermined range.
(4) The compressed progressive wave generated in the ink in the compressing chamber by the electrostrictive vibrator propagates toward the side of the nozzles and toward the side of supplying the ink. Therefore, the size of ink particles injected from the nozzle is also affected by the ratio of a fluid impedance from the compressing chamber to the tip of the nozzle to a fluid impedance from the compressing chamber to the ink reservoir. Accordingly, to reduce the deviation in size of ink particles injected from each of the nozzles, it is necessary to make constant the fluid impedances of the individual nozzles or, in other words, it is necessary to set constant the cross-sectional area and the length of the nozzles.